Fiber-reinforced composites are relatively brittle compared to conventional ductile metal alloys, such as stainless steel and aluminum. Yielding of ductile metals usually reduces the stress concentration around bolt holes so that there is only a loss of area, with no stress concentration at ultimate load on the remaining section at the joints. With composites, however, there is no relief at all from the elastic stress concentration, and catastrophic failure usually results without much warning. Even for small defects in composite structures, the stress-concentration relief is far from complete, although the local disbanding between the fibers and resin matrix and local intraply and interply splitting close to the hole edge does locally alleviate the most severe stress concentrations. Since the stress resistant capability of bolted and riveted joints in composite materials is often unacceptably low, such laminates can never be loaded to levels suggested by the ultimate tensile strength of the laminated composite itself.
It is recognized that the strength of a composite structure with both loaded and unloaded holes depends only slightly on the fiber pattern. Indeed, throughout the range of fiber patterns surrounding laminated structures, the bearing strength and gross-section strengths are almost constant, which simplifies the design process.
The design and analysis of bolted or riveted joints in fibrous composites remains very much an art because of the need to rely on empirical correction factors in some form or another. Mechanically fastened joints differ from bonded composite joints because the presence of holes insures that the joint strength never exceeds the local laminate strength. Indeed, after years of research and development, it appears that only the most carefully designed bolted composite joints will be even half as strong as the basic laminate. Simpler bolted joint configurations will typically attain no more than about a third of the laminate strength. However, because thick composite laminates are often impossible or impractical to adhesively bond or repair, there is a continued need for bolted composite structures.
Since bolted composite structural joints are so brittle, it is very important to calculate accurately the load sharing between fasteners and to identify the most critically loaded one. Bolted joints of composite materials are known to experience many modes of failure, including tension failure, shearout failure, bolts pulling through laminate failure, cleavage tension failure, bearing failure, cutting, impact and bolt failure. See COMPOSITES, Engineered Materials Handbook V1.1, pp. 479–495 (1987), which is hereby incorporated by reference.
The use of local softening strips and pad-ups, has been known to alleviate some of the stress concentrations with respect to basic laminate structures. However, such an approach is not without drawbacks, since these modifications leave the structure outside the locally protected areas with little, if any, damage tolerance because the higher operating strain permitted by the softening strips and pad-ups severely limits the opportunity to perform repairs, which limits the number of situations in which such an approach is practical.
Accordingly, there remains a need for improving the failure resistence of composite structures. In addition, laminate composite technology needs to improve upon the existing design structures to minimize failures associated with shear, bearing, cutting and impact forces.